In recent years, image pickup apparatuses (digital cameras, and/or color scanners, etc.) and image processing softwares which are oriented to consumer have been popularized. Users who themselves edit, by image processing software, image generated by picking up an image of object by the image pick-up apparatus are being increased. Moreover, request with respect to picture quality of image which has been picked up by the image pick-up apparatus has become strong. Ratio of users who mention that picture quality is high as main condition in purchasing the image pick-up apparatus is being increased. Here, general configuration of the image pick-up apparatus will be described below.
In the image pick-up apparatus, there is used, e.g., color filter 1 for three primary colors of RGB as shown in FIG. 1. In this example, color filter 1 is constituted by the so-called Bayer arrangement with four filters in total of two G filters serving to allow only light of green (G) to be transmitted therethrough, one R filter serving to allow only light of red (R) to be transmitted therethrough and one B filter serving to allow only light of blue (B) to be transmitted therethrough being as minimum unit as indicated by single dotted lines of FIG. 1.
FIG. 2 is a block diagram showing a configuration example of a signal processing unit 11 for implementing various processing to RGB signals which have been acquired by CCDs (Charge Coupled Devices) having the RGB color filter 1.
An offset correction processing unit 21 serves to remove offset component included in an image signal which has been delivered from a front end 13 for implementing a predetermined processing to signals which have been acquired by the CCD image pick-up devices to output an image signal thus obtained to a white balance correction processing unit 22. The white balance correction processing unit 22 serves to correct balance of respective colors on the basis of color temperature of the image signal which has been delivered from the offset correction processing unit 21 and differences of sensitivities of respective filters of the color filter 1. The color signal acquired after undergone correction by the white balance correction processing unit 22 is outputted to a gamma (γ)-correction processing unit 23. The gamma-correction processing unit 23 performs gamma-correction with respect to a signal which has been delivered from the white balance correction processing unit 22 to output a signal thus acquired to a vertical direction simultaneous-izing (synchronizing) processing unit 24. At the vertical direction simultaneous-izing processing unit 24, delay elements are provided. Shifts of times in vertical direction of signal which have been delivered from the gamma-correction processing unit 23 are simultaneous-ized (synchronized).
A RGB signal generation processing unit 25 performs interpolation processing for interpolating color signal delivered from the vertical direction simultaneous-izing processing unit 24 into phase of the same space, noise rejection (removal) processing for rejecting or removing noise component of signal, filtering processing for limiting signal band, and high frequency band correction processing for correcting high frequency band component of the signal band, etc. to output RGB signals thus obtained to a luminance signal generation processing unit 26, and a color difference signal generation processing unit 27.
The luminance signal generation processing unit 26 serves to synthesize, at a predetermined synthesis ratio, RGB signals which have been delivered from the RGB signal generation processing unit 25 to generate a luminance signal (Y). The color difference signal generation processing unit 27 serves to similarly synthesize, at a predetermined synthesis ratio, RGB signals which have been delivered from the RGB signal generation processing unit 25 to generate color difference signals (Cb, Cr). The luminance signal (Y) generated by the luminance signal generation processing unit 26 and the color difference signals (Cb, Cr) generated by the color difference signal generation processing unit 27 are outputted to a monitor provided at the outside of the signal processing unit 11, for example.
In a manner as stated above, it is generally performed to implement gamma-processing to an original signal and to conduct thereafter image processing (linear matrix processing) by linear transform processing.
In the image pick-up apparatus as described above, there are instances where since when image of object is picked up to generate image, way of seeing varies depending upon visual environment at the time of observation, reproduction into desired color may not be performed. This is the phenomenon taking place in the case where color rendering characteristics of light source at the time of image pick-up operation (hereinafter referred to as image pick-up light source) and light source at the time of observation (hereinafter referred to an observation light source) are greatly different from each other. In view of the above, there is proposed, in the Japanese Patent Application Laid Open No. 2002-142231 publication, etc., a method of satisfactorily performing color reproduction even in the case where image is reproduced by the observation light source having color rendering characteristic different from that of the image pick-up light source. Moreover, characteristics of spectral sensitivities are shown in FIGS. 3 and 4. Curve L1 of FIG. 3 indicates spectral sensitivity of R, curve L2 indicates spectral sensitivity of G, and curve L3 indicates spectral sensitivity of B. Further, curve L11 of FIG. 4 indicates spectral sensitivity of R, curve L12 indicates spectral sensitivity of G, and curve L13 indicates spectral sensitivity of B.
On the other hand, in the image pick-up apparatus as described above, addition of the linear matrix processing is performed, and addition of color filter of the image pick-up device is further performed, thereby making it possible to dramatically improve color reproduction characteristic (reproducibility). In this instance, when coefficients of a linear matrix used is determined so that color difference is simply minimized, there are instances where noise may be increased as compared to the prior art.